Add a nix-based CI workflow

Author: neilmayhew

.github/workflows/nix-build.yml

@@ -0,0 +1,40 @@
+name: Nix Build
+
+on:
+  push:
+    branches: [develop, main, master]
+  pull_request:
+    branches: [develop, main, master]
+  workflow_dispatch:  # For manual triggering
+  schedule:
+  - cron: "3 14 15 * *"  # 14:03 on the 15th of every month (UTC)
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        compiler: ['ghc94', 'ghc96', 'ghc98', 'ghc910', 'ghc912']
+      fail-fast: false
+
+    steps:
+
+    - name: Checkout
+      uses: actions/checkout@v4
+
+    - name: Install nix
+      uses: cachix/install-nix-action@v31
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+
+    - name: Enable cachix
+      uses: cachix/cachix-action@v16
+      with:
+        name: neil-mayhew
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+
+    - name: Build with ${{ matrix.compiler }}
+      id: build
+      run: nix build -f shell.nix
+        --argstr compiler ${{ matrix.compiler }}
+        --print-out-paths | cachix push neil-mayhew